A. Field of the Invention
This invention relates to a process for applying a thin coating or layer of tin or tin alloy on a composite structure which has one surface composed of an aluminum base metal (i.e., aluminum or an alloy of aluminum) and another surface composed of a ferrous base metal) (i.e., iron or an alloy of iron). More specifically, it concerns a method of concurrently applying a layer or coating of tin or a tin alloy on the exposed surface of a bearing structure which includes a steel substrate which carries or supports an aluminum base metal bearing surface by treating the surface of the bearing with a mineral acid containing either fluoride ions, fluoride containing ions or mixtures thereof and then immersing the bearing in an aqueous plating bath containing a mineral acid, a source of either fluoride ions, fluoride containing ions or mixtures thereof and a source of stannous ions with the stannous ions being present in an amount ranging from about 1 to about 75 grams per liter.
B. Description of the Prior Art
The deposition of a thin coating or plating of tin on the surface of bearings composed of aluminum or aluminum alloys in order to impart a pleasing appearance thereto or to provide corrosion protection therefore, or to provide a "run-in" surface thereon is well known in the art.
Specifically, it is common practice to apply a thin coating of tin to the surface of a bearing by means of electrodeposition to achieve such results. While this technique has met with success, there are certain inherent disadvantages associated therewith. For example, an electrical power source must be provided and the parts must be properly oriented in the plating bath in order to obtain a satisfactory deposit thereon. In addition, an electrodeposition technique also suffers from the fact that it is difficult to apply thin layers of metal to a structure having a complicated surface configuration. Accordingly, while this technique finds use as a means of applying a thin layer of tin to the surface of a bearing structure it suffers from certain inherent limitations.
Another well known technique for coating the surface of a bearing with tin is the so-called immersion plating process. In this process, metal is deposited from its salt on the surface of the bearing without the aid of an outside source of electrical current or of chemical reducing agents. This process is especially appealing in that thin coatings of uniform thickness can be readily applied to a structure having a complicated surface configuration. Immersion tin plating baths are either alkaline or acidic. While both types of baths can be used to deposit tin on the surface of aluminum or aluminum alloy, none of the heretofore known baths can be used to satisfactorily apply a thin tin coating to a composite structure having one surface composed of an aluminum base metal and another surface composed of a ferrous base metal. Specifically, alkaline tin immersion baths do not coat both the aluminum and ferrous base metals, but only the aluminum base metal. In addition, the adhesion of tin to the aluminum base metal is generally poor and tends to blister and peel. Various immersion acid tin plating baths have been used quite successfully to deposit a thin layer of tin on a structure which is all aluminum or an alloy thereof, however, when such plating baths are employed to apply a thin layer of tin to a composite structure having a surface composed of an aluminum base metal and another surface composed of a ferrous base metal, the tin deposit so obtained does not exhibit good adhesion to both metal surfaces. To overcome this problem, it is known in the art to apply tin to the surface of the aluminum base metal by immersion plating techniques and to apply tin to the ferrous base metal surface by electrodeposition. Obviously, this procedure is quite involved and is to be avoided, if possible.
Another well known technique for depositing a thin layer of tin on a surface is the so-called contact plating technique. In this technique, the article to be tinned is usually in direct contact with a piece of tin or zinc in the solution. The contact process is in effect an electrolytic method, with the outside source of currents being replaced by a galvanic couple. However, when conventional contact plating baths are utilized to apply tin to a composite structure having a ferrous base metal surface and an aluminum base metal surface, the coating obtained on the aluminum base metal surface is generally of a very poor quality. This is due to the fact that conventional contact plating baths are designed to produce the desired coating on only the more noble metals.
In order to overcome or at least significantly minimize the hereinbefore discussed difficulties applicants have developed a unique plating process which will be described hereinafter in more detail.